A network may use both optical fiber and coaxial cable (coax) for respective links. For example, the portions of the network that use optical fiber may be implemented using the Ethernet Passive Optical Network (EPON) protocol, and the EPON protocol may be extended over coaxial cable plants. The EPON protocol as implemented over coax is called EPoC. The physical layer (PHY) of such a network may use orthogonal frequency-division multiplexing (OFDM) and/or orthogonal frequency division multiple access (OFDMA).
Signals transmitted in a particular cable plant are distorted by micro-reflections that occur in the coax communication channel. The delay and amplitude of these micro-reflections depend on the specific cable plant. Also, in a system using frequency-division duplexing (FDD), the amplitude and the delay spread of the micro-reflections may be different for the different frequencies that are used for downstream and upstream communications. For example, the amplitude and delay spread of the micro-reflections may be greater at the low frequencies used for upstream transmission than at the higher frequencies used for downstream transmission.
In an OFDM PHY before each symbol there is a cyclic prefix to deal with the impact of the delay spread of the communication channel, which in a cable plant is due to the micro-reflections. If the OFDM cyclic prefix is long, however, it leads to either long OFDM symbols or high cyclic prefix overhead.
Accordingly, there is a need for efficient cycle prefix schemes in OFDM systems with coax communication channels.
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